Conventionally, a film screen system formed by combining an intensifying screen with a radiographic film has been widely used for X-ray imaging aimed at medical diagnosis. A film is designed to enhance the contrast in a density range in which a portion to be diagnosed can be easily observed when a film image obtained by X-ray imaging is observed on a film viewer. However, deviations from the designed imaging conditions tend to cause overexposure or underexposure, resulting in affecting diagnosis.
Recently, an X-ray digital imaging apparatus has begun to be used, which employs a flat panel detector (to be referred to as an FPD hereinafter) which converts X-rays into electrical signals proportional to the intensities of the X-rays. According to this imaging apparatus, an image of a subject to be examined is obtained by imaging by an X-ray detector having a large area. A region irradiated with X-rays is then detected from the entire image region by an image processing technique, and a characteristic feature of the image is extracted, thereby performing density adjustment. This can therefore solve the conventional problem concerning exposure adjustment (see, for example, Japanese Patent Publication No. 7-52284). In addition, since a designated region is extracted from the entire image region, invalid digital information can be deleted, and the amount of information stored can be reduced. This apparatus also has a function for improving diagnostic performance by image processing such as image enhancement.
The above radiation field recognition is, however, implemented by a program, and 100% success for radiation field recognition cannot be expected unless an imaging method based on the X-ray dose, the position of a subject to be imaged, and the like intended by the program is executed. That is, when imaging is performed under exceptional conditions, a crop region must be manually corrected.
When a crop region is to be designated in accordance with an instruction from an operator as described above, an excessively small crop region may be designated due to human errors in operating a mouse and touch panel. The operator often gives his/her attention to the density of an image. If, therefore, the density is stable by chance, and the operator terminates the imaging operation without any concern for the validity of the crop region, an invalid image may be transmitted to an external printer or image storage device, or an image display device.